Greg writes: My array uses a FET source follower on each element, for the exact
reasons that Tom cites. The series capacitance of a 3m element on 160m
is around 20pF, a reactance of 4 kohm. It's much easier to match 8 of these
using a high Z amp, in my opinion.
I also use Fet source followers and larger elements with about 60pf in
my array for similar reasons. In my opinion I get a more stable phase shift and
voltage gain in the antenna/amplifier combination for varying environmental
conditions. In addition they have a very constant output impedance to not only
drive cables but to properly load Magic Tee combiners as pointed out by Tom. I
also had to supplement added gain behind my combiners but in front of some
other circuit losses to keep up my array N/F even with the taller elements. I
have a pretty quiet location. YMMV. I wish some of these discussions had
occured earlier during my design efforts. Might not have had to rebuild my
array 4 times before it worked right, Hi.
I have not tried transformers with shortened elements but Ricks and Johns
comments have shown me that a new High angle RX antenna array I was looking at
for early mornings would work really well using one or 2 or 4 transformers etc.
My vertical array is terrible for high angle.
That being said, I believe that as we get better and better at building these
specialty antennas it is these discussions and free exchange of ideas that will
pave the way for greater things. Precisely why I have chosen to share my work
freely. You want more performance in a smaller space, you have to try to look
at every detail. Since I posted my 8 element design a few weeks ago another Ham
has shared with me a solid looking idea to improve the RDF and the bandwidth
and even reduce the physical size. It strikes me as having a few more
challenges though. I say Hoo Hah bring it on.
Lee K7TJR
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